Coordinated top electrode - drive electrode voltages for switching optical state of electrophoretic displays using positive and negative voltages of different magnitudes

ABSTRACT

A system for simplified driving of electrophoretic media using a positive and a negative voltage source, where the voltage sources have different magnitudes, and a controller that cycles the top electrode between the two voltage sources and ground while coordinating driving at least two drive electrodes opposed to the top electrode. The resulting system can achieve roughly the same color states as compared to supplying each drive electrode with six independent drive levels and ground. Thus, the system simplifies the required electronics with only marginal loss in color gamut. The system is particularly useful for addressing an electrophoretic medium including four sets of different particles, e.g., wherein three of the particles are colored and subtractive and one of the particles is light-scattering.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 17/474,375, filed Sep. 14, 2021, which claims priority to U.S.Provisional Application No. 63/078,829, filed. Sep. 15, 2020. Thisapplication additionally claims priority to U.S. Provisional PatentApplication No. 63/320,524, filed Mar. 16, 2022. All patents andpublications disclosed herein are incorporated by reference in theirentireties.

BACKGROUND

An electrophoretic display (EPD) changes color by modifying the positionof a charged colored particle with respect to a light-transmissiveviewing surface. Such electrophoretic displays are typically referred toas “electronic paper” or “ePaper” because the resulting display has highcontrast and is sunlight-readable, much like ink on paper.Electrophoretic displays have enjoyed widespread adoption in eReaders,such as the AMAZON KINDLE® because the electrophoretic displays providea book-like reading experience, use little power, and allow a user tocarry a library of hundreds of books in a lightweight handheld device.

For many years, electrophoretic displays included only two types ofcharged color particles, black and white. (To be sure, “color” as usedherein includes black and white.) The white particles are often of thelight scattering type, and comprise, e.g., titanium dioxide, while theblack particle are absorptive across the visible spectrum, and maycomprise carbon black, or an absorptive metal oxide, such as copperchromite. In the simplest sense, a black and white electrophoreticdisplay only requires a light-transmissive electrode at the viewingsurface, a back electrode, and an electrophoretic medium includingoppositely charged white and black particles. When a voltage of onepolarity is provided, the white particles move to the viewing surface,and when a voltage of the opposite polarity is provided the blackparticles move to the viewing surface. If the back electrode includescontrollable regions (pixels)—either segmented electrodes or an activematrix of pixel electrodes controlled by transistors—a pattern can bemade to appear electronically at the viewing surface. The pattern canbe, for example, the text to a book.

More recently, a variety of color option have become commerciallyavailable for electrophoretic displays, including three-color displays(black, white, red; black white, yellow), and four color displays(black, white, red, yellow). Similar to the operation of black and whiteelectrophoretic displays, electrophoretic displays with three or fourreflective pigments operate similar to the simple black and whitedisplays because the desired color particle is driven to the viewingsurface. The driving schemes are far more complicated than only blackand white, but in the end, the optical function of the particles is thesame.

Advanced Color electronic Paper (ACeP™) also includes four particles,but the cyan, yellow, and magenta particles are subtractive rather thanreflective, thereby allowing thousands of colors to be produced at eachpixel. The color process is functionally equivalent to the printingmethods that have long been used in offset printing and ink-jetprinters. A given color is produced by using the correct ratio of cyan,yellow, and magenta on a bright white paper background. In the instanceof ACeP, the relative positions of the cyan, yellow, magenta and whiteparticles with respect to the viewing surface will determine the colorat each pixel. While this type of electrophoretic display allows forthousands of colors at each pixel, it is critical to carefully controlthe position of each of the (50 to 500 nanometer-sized) pigments withina working space of about 10 to 20 micrometers in thickness. Obviously,variations in the position of the pigments will result in incorrectcolors being displayed at a given pixel. Accordingly, exquisite voltagecontrol is required for such a system. More details of this system areavailable in the following U.S. Patents, all of which are incorporatedby reference in their entireties: U.S. Pat. Nos. 9,361,836, 9,921,451,10,276,109, 10,353,266, 10,467,984, and 10,593,272.

This invention relates to color electrophoretic displays, especially,but not exclusively, to electrophoretic displays capable of renderingmore than two colors using a single layer of electrophoretic materialcomprising a plurality of colored particles, for example white, cyan,yellow, and magenta particles. In some instances two of the particleswill be positively-charged, and two particles will benegatively-charged. In some instances three of the particles will bepositively-charged, and one particle will be negatively-charged. In someinstances, one positively-charged particle will have a thick polymershell and one negatively-charged particle has a thick polymer shell.

The term gray state is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate gray state would actually be pale blue.Indeed, as already mentioned, the change in optical state may not be acolor change at all. The terms black and white may be used hereinafterto refer to the two extreme optical states of a display, and should beunderstood as normally including extreme optical states which are notstrictly black and white, for example the aforementioned white and darkblue states.

The terms bistable and bistability are used herein in their conventionalmeaning in the art to refer to displays comprising display elementshaving first and second display states differing in at least one opticalproperty, and such that after any given element has been driven, bymeans of an addressing pulse of finite duration, to assume either itsfirst or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called multi-stable rather than bistable,although for convenience the term bistable may be used herein to coverboth bistable and multi-stable displays.

The term impulse, when used to refer to driving an electrophoreticdisplay, is used herein to refer to the integral of the applied voltagewith respect to time during the period m which the display is driven.

A particle that absorbs, scatters, or reflects light, either in a broadband or at selected wavelengths, is referred to herein as a colored orpigment particle. Various materials other than pigments (in the strictsense of that term as meaning insoluble colored materials) that absorbor reflect light, such as dyes or photonic crystals, etc., may also beused in the electrophoretic media and displays of the present invention.

Particle-based electrophoretic displays have been the subject of intenseresearch and development for a number of years. In such displays, aplurality of charged particles (sometimes referred to as pigmentparticles) move through a fluid under the influence of an electricfield. Electrophoretic displays can have attributes of good brightnessand contrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., Electrical toner movement for electronicpaper-like display, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., etal., Toner display using insulative particles charged triboelectrically,IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and7,236,291. Such gas-based electrophoretic media appear to be susceptibleto the same types of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous suspendingfluids as compared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thesepatents and applications include:

-   -   (a) Electrophoretic particles, fluids and fluid additives; see        for example U.S. Pat. Nos. 7,002,728 and 7,679,814;    -   (b) Capsules, hinders and encapsulation processes; see for        example U.S. Pat. Nos. 6,922,276 and 7,411,719;    -   (c) Microcell structures, wall materials, and methods of forming        microcells; see for example U.S. Pat. Nos. 7,072,095 and        9,279,906;    -   (d) Methods for filling and sealing microcells; see for example        U.S. Pat. Nos. 7,144,942 and 7,715,088;    -   (e) Films and sub-assemblies containing electro-optic materials;        see for example U.S. Pat. Nos. 6,982,178 and 7,839,564;    -   (f) Backplanes, adhesive layers and other auxiliary layers and        methods used in displays; see for example U.S. Pat. Nos.        7,116,318 and 7,535,624;    -   (g) Color formation color adjustment; see for example U.S. Pat.        Nos. 6,017,584; 6,545,797; 6,664,944; 6,788,452; 6,864,875;        6,914,714; 6,972,893; 7,038,656; 7,038,670; 7,046,228;        7,052,571; 7,075,502; 7,167,155; 7,385,751; 7,492,505;        7,667,684; 7,684,108; 7,791,789; 7,800,813; 7,821,702;        7,839,564; 7,910,175; 7,952,790; 7,956,841; 7,982,941;        8,040,594; 8,054,526; 8,098,418; 8,159,636; 8,213,076;        8,363,299; 8,422,116; 8,441,714; 8,441,716; 8,466,852;        8,503,063; 8,576,470; 8,576,475; 8,593,721; 8,605,354;        8,649,084; 8,670,174; 8,704,756; 8,717,664; 8,786,935;        8,797,634; 8,810,899; 8,830,559; 8,873,129; 8,902,153;        8,902,491; 8,917,439; 8,964,282; 9,013,783; 9,116,412;        9,146,439; 9,164,207; 9,170,467; 9,170,468; 9,182,646;        9,195,111; 9,199,441; 9,268,191; 9,285,649; 9,293,511;        9,341,916; 9,360,733; 9,361,836; 9,383,623; and 9,423,666; and        U.S. Patent Applications Publication Nos. 2008/0043318;        2008/0048970; 2009/0225398; 2010/0156780; 2011/0043543;        2012/0326957; 2013/0242378; 2013/0278995; 2014/0055840;        2014/0078576; 2014/0340430; 2014/0340736; 2014/0362213;        2015/0103394; 2015/0118390; 2015/0124345; 2015/0198858;        2015/0234250; 2015/0268531; 2015/0301246; 2016/0011484;        2016/0026062; 2016/0048054; 2016/0116816; 2016/0116818; and        2016/0140909;    -   (h) Methods for driving displays; see for example U.S. Pat. Nos.        5,930,026; 6,445,489; 6,504,524; 6,512,354; 6,531,997;        6,753,999; 6,825,970; 6,900,851; 6,995,550; 7,012,600;        7,023,420; 7,034,783; 7,061,166; 7,061,662; 7,116,466;        7,119,772; 7,177,066; 7,193,625; 7,202,847; 7,242,514;        7,259,744; 7,304,787; 7,312,794; 7,327,511; 7,408,699;        7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251;        7,602,374; 7,612,760; 7,679,599; 7,679,813; 7,683,606;        7,688,297; 7,729,039; 7,733,311; 7,733,335; 7,787,169;        7,859,742; 7,952,557; 7,956,841; 7,982,479; 7,999,787;        8,077,141; 8,125,501; 8,139,050; 8,174,490; 8,243,013;        8,274,472; 8,289,250; 8,300,006; 8,305,341; 8,314,784;        8,373,649; 8,384,658; 8,456,414; 8,462,102; 8,514,168;        8,537,105; 8,558,783; 8,558,785; 8,558,786; 8,558,855;        8,576,164; 8,576,259; 8,593,396; 8,605,032; 8,643,595;        8,665,206; 8,681,191; 8,730,153; 8,810,525; 8,928,562;        8,928,641; 8,976,444; 9,013,394; 9,019,197; 9,019,198;        9,019,318; 9,082,352; 9,171,508; 9,218,773; 9,224,338;        9,224,342; 9,224,344; 9,230,492; 9,251,736; 9,262,973;        9,269,311; 9,299,294; 9,373,289; 9,390,066; 9,390,661; and        9,412,314; and U.S. Patent Applications Publication Nos.        2003/0102858; 2004/0246562; 2005/0253777; 2007/0091418;        2007/0103427; 2007/0176912; 2008/0024429; 2008/0024482;        2008/0136774; 2008/0291129; 2008/0303780; 2009/0174651;        2009/0195568; 2009/0322721; 2010/0194733; 2010/0194789;        2010/0220121; 2010/0265561; 2010/0283804; 2011/0063314;        2011/0175875; 2011/0193840; 2011/0193841; 2011/0199671;        2011/0221740; 2012/0001957; 2012/0098740; 2013/0063333;        2013/0194250; 2013/0249782; 2013/0321278; 2014/0009817;        2014/0085355; 2014/0204012; 2014/0218277; 2014/0240210;        2014/0240373; 2014/0253425; 2014/0292830; 2014/0293398;        2014/0333685; 2014/0340734; 2015/0070744; 2015/0097877;        2015/0109283; 2015/0213749; 2015/0213765; 2015/0221257;        2015/0262255; 2015/0262551; 2016/0071465; 2016/0078820;        2016/0093253; 2016/0140910; and 2016/0180777 (these patents and        applications may hereinafter be referred to as the MEDEOD        (MEthods for Driving Electro-optic Displays) applications);    -   (i) Applications of displays; see for example U.S. Pat. Nos.        7,312,784 and 8,009,348; and    -   (j) Non-electrophoretic displays, as described in U.S. Pat. No.        6,241,921; and U.S. Patent Applications Publication Nos.        2015/0277160; and U.S. Patent Application Publications Nos.        2015/0005720 and 2016/0012710.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,866,760. Accordingly, for purposes of the presentapplication, such polymer-dispersed electrophoretic media are regardedas sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called microcellelectrophoretic display. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called shutter mode in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Other types of electro-optic displays may also be capable of operatingin shutter mode. Electro-optic media operating in shutter mode can beused in multi-layer structures for full color displays; in suchstructures, at least one layer adjacent the viewing surface of thedisplay operates in shutter mode to expose or conceal a second layermore distant from the viewing surface.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word printing is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

As indicated above most simple prior art electrophoretic mediaessentially display only two colors. Such electrophoretic media eitheruse a single type of electrophoretic particle having a first color in acolored fluid having a second, different color (in which case, the firstcolor is displayed when the particles lie adjacent the viewing surfaceof the display and the second color is displayed when the particles arespaced from the viewing surface), or first and second types ofelectrophoretic particles having differing first and second colors in anuncolored fluid (in which case, the first color is displayed when thefirst type of particles lie adjacent the viewing surface of the displayand the second color is displayed when the second type of particles lieadjacent the viewing surface). Typically the two colors are black andwhite. If a full color display is desired, a color filter array may bedeposited over the viewing surface of the monochrome (black and white)display.

Displays with color filter arrays rely on area sharing and colorblending to create color stimuli. The available display area is sharedbetween three or four primary colors such as red/green/blue (RGB) orred/green/blue/white (RGBW), and the filters can be arranged inone-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Otherchoices of primary colors or more than three primaries are also known inthe art. The three (in the case of RGB displays) or four (in the case ofRGBW displays) sub-pixels are chosen small enough so that at theintended viewing distance they visually blend together to a single pixelwith a uniform color stimulus (‘color blending’). The inherentdisadvantage of area sharing is that the colorants are always present,and colors can only be modulated by switching the corresponding pixelsof the underlying monochrome display to white or black (switching thecorresponding primary colors on or off). For example, in an ideal RGBWdisplay, each of the red, green, blue and white primaries occupy onefourth of the display area (one sub-pixel out of four), with the whitesub-pixel being as bright as the underlying monochrome display white,and each of the colored sub-pixels being no lighter than one third ofthe monochrome display white. The brightness of the white color shown bythe display as a whole cannot be more than one half of the brightness ofthe white sub-pixel (white areas of the display are produced bydisplaying the one white sub-pixel out of each four, plus each coloredsub-pixel in its colored form being equivalent to one third of a whitesub-pixel, so the three colored sub-pixels combined contribute no morethan the one white sub-pixel). The brightness and saturation of colorsis lowered by area-sharing with color pixels switched to black. Areasharing is especially problematic when mixing yellow because it islighter than any other color of equal brightness, and saturated yellowis almost as bright as white. Switching the blue pixels (one fourth ofthe display area) to black makes the yellow too dark.

A commonly used system for quantifying the color characteristics of adisplay, including both brightness and hue is the CIELAB system, whichassigns color coordinate values (i.e., L*, a*, b*) corresponding tocolors displayed by typical color reflective display devices under a CIEstandard illuminant D65 (e.g., with color temperature 6500K). L*represents lightness from black to white on a scale of zero to 100,while a* and b* represent chromaticity with no specific numeric limits.Negative a* corresponds with green, positive a* corresponds with red,negative b* corresponds with blue and positive b* corresponds withyellow. L* can be converted to reflectance with the following formula:L*=116(R/R₀)^(1/3)−16, where R is the reflectance and R₀ is a standardreflectance value.

U.S. Pat. Nos. 8,576,476 and 8,797,634 describe multicolorelectrophoretic displays having a single back plane comprisingindependently addressable pixel electrodes and a common,light-transmissive front electrode. The common, light-transmissive frontelectrode is also known as the top electrode. Between the back plane andthe front electrode is disposed a plurality of electrophoretic layers.Displays described in these applications are capable of rendering any ofthe primary colors (red, green, blue, cyan, magenta, yellow, white andblack) at any pixel location. However, there are disadvantages to theuse of multiple electrophoretic layers located between a single set ofaddressing electrodes. The electric field experienced by the particlesin a particular layer is lower than would be the case for a singleelectrophoretic layer addressed with the same voltage. In addition,optical losses in an electrophoretic layer closest to the viewingsurface (for example, caused by light scattering or unwanted absorption)may affect the appearance of images formed in underlying electrophoreticlayers.

Attempts have been made to provide full-color electrophoretic displaysusing a single electrophoretic layer. For example, U.S. Pat. No.8,917,439 describes a color display comprising an electrophoretic fluidthat comprises one or two types of pigment particles dispersed in aclear and colorless or colored solvent, the electrophoretic fluid beingdisposed between a common electrode and a plurality of pixel or drivingelectrodes. The driving electrodes are arranged to expose a backgroundlayer. U.S. Pat. No. 9,116,412 describes a method for driving a displaycell filled with an electrophoretic fluid comprising two types ofcharged particles carrying opposite charge polarities and of twocontrast colors. The two types of pigment particles are dispersed in acolored solvent or in a solvent with non-charged or slightly chargedcolored particles dispersed therein. The method comprises driving thedisplay cell to display the color of the solvent or the color of thenon-charged or slightly charged colored particles by applying a drivingvoltage that is about 1 to about 20% of the full driving voltage. U.S.Pat. Nos. 8,717,664 and 8,964,282 describe an electrophoretic fluid, anda method for driving an electrophoretic display. The fluid comprisesfirst, second and third type of pigment particles, all of which aredispersed in a solvent or solvent mixture. The first and second types ofpigment particles carry opposite charge polarities, and the third typeof pigment particles has a charge level being less than about 50% of thecharge level of the first or second type. The three types of pigmentparticles have different levels of threshold voltage, or differentlevels of mobility, or both. None of these patent applications disclosefull color display in the sense in which that term is used below, thatis capable of achieving at least eight independent colors (white, red,green, blue, cyan, yellow, magenta, and black).

SUMMARY

Disclosed herein are improved methods of driving full colorelectrophoretic displays and full color electrophoretic displays usingthese drive methods. In one aspect, the invention involves a. colorelectrophoretic display including a light-transmissive electrode at aviewing surface, a backplane including an array of thin film transistorscoupled to pixel electrodes, wherein each thin film transistorcomprising a layer of a metal oxide semiconductor, and a colorelectrophoretic medium disposed between the light-transmissive electrodeand the backplane. The color electrophoretic medium includes (a) afluid, (b) a plurality of first and a plurality of second particlesdispersed in the fluid, the first and second particles bearing chargesof opposite polarity, the first particle being a light-scatteringparticle and the second particle having one of the subtractive primarycolors, and (c) a plurality of third and a plurality of fourth particlesdispersed in the fluid, the third and fourth particles bearing chargesof opposite polarity, the third and fourth particles each having asubtractive primary color different from each other and from the secondparticles.

In some embodiments, a first electric field required to separate anaggregate formed by the third and the fourth types of particles isgreater than a second electric field required to separate an aggregateformed from any other two types of particles. In some embodiments, atleast two of the second, third and fourth particles arenon-light-scattering. In some embodiments, the first particles are whiteand the second, third and fourth particles are non-light-scattering. Insome embodiments, the first and third particles are negatively chargedand the second and fourth particles are positively charged. In someembodiments, the first, second, third and fourth particles arerespectively white, cyan, yellow and magenta in color, with the whiteand yellow particles being negatively charged and the magenta and cyanparticles positively charged. In some embodiments, the yellow, magentaand cyan pigments exhibit diffuse reflectances at 650, 550 and 450 nm,respectively, measured over a black background, of less than 2.5% whenthe pigment is approximately isotropically distributed at 15% by volumein a layer of thickness 1 μm comprising the pigment and a liquid ofrefractive index less than 1.55. In some embodiments, the liquid is anon-polar liquid having a dielectric constant less than about 5. In someembodiments, the fluid has have dissolved or dispersed therein a polymerhaving a number average molecular weight in excess of about 20,000 andbeing essentially non-absorbing on the particles. In some embodiments,the metal oxide semiconductor is indium gallium zinc oxide (IGZO). Theinventions above may be incorporated into an electronic book reader,portable computer, tablet computer, cellular telephone, smart card,sign, watch, shelf label or flash drive.

In another aspect, a color electrophoretic display including acontroller, a light-transmissive electrode at a viewing surface, and abackplane including an array of thin film transistors coupled to pixelelectrodes, each thin film transistor comprising a layer of a metaloxide semiconductor. A color electrophoretic medium is disposed betweenthe light-transmissive electrode and the backplane, and the colorelectrophoretic medium includes (a) a fluid, (b) a plurality of firstand a plurality of second particles dispersed in the fluid, the firstand second particles bearing charges of opposite polarity, the firstparticle being a light-scattering particle and the second particlehaving one of the subtractive primary colors, and (c) a plurality ofthird and a plurality of fourth particles dispersed in the fluid, thethird and fourth particles bearing charges of opposite polarity, thethird and fourth particles each having a subtractive primary colordifferent from each other and from the second particles. The controlleris configured to provide a plurality of driving voltages to the pixelelectrodes such that white, yellow, red, magenta, blue, cyan, green, andblack can be displayed at each pixel electrode while keeping thelight-transmissive electrode at a constant voltage. In some embodiments,the controller is configured to provide a voltage of greater than 25Volts and less than −25 Volts to the pixel electrodes. In someembodiments, the controller is configured to additionally provide avoltage between 25 V and 0V and a voltage between −25V and 0V. In someembodiments, the metal oxide semiconductor is indium gallium zinc oxide(IGZO).

In another aspect, a color electrophoretic display including acontroller, a light-transmissive electrode at a viewing surface, abackplane electrode, and a color electrophoretic medium disposed betweenthe light-transmissive electrode and the backplane electrode. The colorelectrophoretic medium includes (a) a fluid, (b) a plurality of firstand a plurality of second particles dispersed in the fluid, the firstand second particles bearing charges of opposite polarity, the firstparticle being a light-scattering particle and the second particlehaving one of the subtractive primary colors, and (c) a plurality ofthird and a plurality of fourth particles dispersed in the fluid, thethird and fourth particles bearing charges of opposite polarity, thethird and fourth particles each having a subtractive primary colordifferent from each other and from the second particles. The controlleris configured to provide a first high voltage and a first low voltage tothe light transmissive electrode, and a second high voltage, a zerovoltage, and a second low voltage to the backplane electrode, such thatthe colors white, yellow, red, magenta, blue, cyan, green, and black canbe displayed at the viewing surface, wherein the magnitude of at leastone of the first high voltage, the first low voltage, the second highvoltage, and the second low voltage are not the same. In someembodiments, the magnitude of the first high voltage and the magnitudeof the second high voltage are the same. In some embodiments, themagnitude of the first low voltage and the magnitude of the second lowvoltage are the same, and the magnitude of the first high voltage andthe magnitude of the first low voltage are not the same.

In another aspect, a color electrophoretic display including acontroller; a light-transmissive electrode at a viewing surface, abackplane electrode, and a color electrophoretic medium disposed betweenthe light-transmissive electrode and the backplane electrode. The colorelectrophoretic medium includes (a) a fluid, (b) a plurality of firstand a plurality of second particles dispersed in the fluid, the firstand second particles bearing charges of opposite polarity, the firstparticle being a light-scattering particle and the second particlehaving one of the subtractive primary colors; and (c) a plurality ofthird and a plurality of fourth particles dispersed in the fluid, thethird and fourth particles bearing charges of opposite polarity, thethird and fourth particles each having a subtractive primary colordifferent from each other and from the second particles. The controlleris configured to cause the colors white, yellow, red, magenta, blue,cyan, green, and black color to be displayed at the viewing surface byproviding one of a plurality of time dependent drive voltages to thebackplane electrode while providing one of the foil owing drive voltageto the light-transmissive electrode 1) a high voltage for time a firsttime, a low voltage for a second time, and a high voltage for a thirdtime, or 2) a low voltage for time a first time, a high voltage for asecond time, and a low voltage for a third time.

In another aspect, a system for driving an electrophoretic medium,comprising an electrophoretic display, a power source capable ofproviding a positive voltage and a negative voltage, where the magnitudeof the positive voltage and the negative voltage are different, and acontroller coupled to the top electrode driver, the first driveelectrode driver, and the second drive electrode driver. Theelectrophoretic medium includes a light-transmissive top electrode at aviewing surface, a first drive electrode, a second drive electrode, andan electrophoretic medium disposed between the top electrode and thefirst and second drive electrodes. The controller is configured toprovide A) in a first frame, the positive voltage to the top electrode,the negative voltage to the first drive electrode, and the positivevoltage to the second drive electrode, B) in a second frame, thenegative voltage to the top electrode, the negative voltage to the firstdrive electrode, and the negative voltage to the second drive electrode,C) in a third frame, the ground voltage to the top electrode, the groundvoltage to the first drive electrode, and the positive voltage to thesecond drive electrode, and D) in a fourth frame, the positive voltageto the top electrode, the positive voltage to the first drive electrode,and the positive voltage to the second drive electrode. In oneembodiment, the controller is configured to further provide E) in afifth frame, the negative voltage to the top electrode, the groundvoltage to the first drive electrode, and the negative voltage to thesecond drive electrode, and F) in a sixth frame, the ground voltage tothe top electrode, the ground voltage to the first drive electrode, andthe ground voltage to the second drive electrode. In one embodiment, theelectrophoretic medium is encapsulated in a plurality of microcapsulesand the microcapsules are dispersed in a polymer binder between the topelectrode and the first and second drive electrodes. In one embodiment,the electrophoretic medium is encapsulated in an array of microcellshaving openings wherein the opening are sealed with a polymer binder,and the array of microcells is disposed between the top electrode andthe first and second drive electrodes. In one embodiment, theelectrophoretic medium comprises a non-polar fluid and four sets ofparticles having different optical properties. In one embodiment, thefirst and second sets of particles bear charges of opposite polarity,the third and fourth sets of particles bear charges of oppositepolarity, the first particle is a light-scattering particle, and thesecond, third, and fourth sets of particles are each a subtractiveprimary color different from each other. In one embodiment, thecontroller is configured to provide combinations of the positivevoltage, the negative voltage, and the ground voltage to the topelectrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can be displayed atthe viewing surface. In one embodiment, the first and second sets ofparticles bear charges of opposite polarity, the third and fourth setsof particles bear the same charge as the second particle, the firstparticle is a light-scattering particle, and the second, third, andfourth sets of particles are each a subtractive primary color differentfrom each other. In one embodiment, the controller is configured toprovide combinations of the positive voltage, the negative voltage, andthe ground voltage to the top electrode and the first drive electrodesuch that the colors white, yellow, red, magenta, blue, cyan, green, andblack can be displayed at the viewing surface. In one embodiment, thepositive voltage is +15V and the negative voltage is −9V. In oneembodiment, the positive voltage is +9V and the negative voltage is−15V.

In another aspect, a system for driving an electrophoretic medium,comprising an electrophoretic display, a power source capable ofproviding a positive voltage and a negative voltage, where the magnitudeof the positive voltage and the negative voltage are different, and acontroller coupled to the top electrode driver, the first driveelectrode driver, and the second drive electrode driver. Theelectrophoretic medium includes a light-transmissive top electrode at aviewing surface, a first drive electrode, a second drive electrode, andan electrophoretic medium disposed between the top electrode and thefirst and second drive electrodes. The controller is configured toprovide A) in a first frame, the positive voltage to the top electrode,the negative voltage to the first drive electrode, and the positivevoltage to the second drive electrode, B) in a second frame, thenegative voltage to the top electrode, the negative voltage to the firstdrive electrode, and the negative voltage to the second drive electrode,C) in a third frame, the ground voltage to the top electrode, the groundvoltage to the first drive electrode, and the ground voltage to thesecond drive electrode, and D) in a fourth frame, the positive voltageto the top electrode, the positive voltage to the first drive electrode,and the positive voltage to the second drive electrode. In oneembodiment, the controller is configured to further provide E) in afifth frame, the negative voltage to the top electrode, the groundvoltage to the first drive electrode, and the negative voltage to thesecond drive electrode, and F) in a sixth frame, the ground voltage tothe top electrode, the ground voltage to the first drive electrode, andthe ground voltage to the second drive electrode. In one embodiment, theelectrophoretic medium is encapsulated in a plurality of microcapsulesand the microcapsules are dispersed in a polymer binder between the topelectrode and the first and second drive electrodes. In one embodiment,the electrophoretic medium is encapsulated in an array of microcellshaving openings wherein the opening are sealed with a polymer binder,and the array of microcells is disposed between the top electrode andthe first and second drive electrodes. In one embodiment, theelectrophoretic medium comprises a non-polar fluid and four sets ofparticles having different optical properties. In one embodiment, thefirst and second sets of particles bear charges of opposite polarity,the third and fourth sets of particles bear charges of oppositepolarity, the first particle is a light-scattering particle, and thesecond, third, and fourth sets of particles are each a subtractiveprimary color different from each other. In one embodiment, thecontroller is configured to provide combinations of the positivevoltage, the negative voltage, and the ground voltage to the topelectrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can be displayed atthe viewing surface. In one embodiment, the first and second sets ofparticles bear charges of opposite polarity, the third and fourth setsof particles bear the same charge as the second particle, the firstparticle is a light-scattering particle, and the second, third, andfourth sets of particles are each a subtractive primary color differentfrom each other. In one embodiment, the controller is configured toprovide combinations of the positive voltage, the negative voltage, andthe ground voltage to the top electrode and the first drive electrodesuch that the colors white, yellow, red, magenta, blue, cyan, green, andblack can be displayed at the viewing surface. In one embodiment, thepositive voltage is +15V and the negative voltage is −9 V. In oneembodiment, the positive voltage is +9V and the negative voltage is−15V.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a schematic cross-section showing an embodiment of anencapsulated electrophoretic display suitable for use with the methodsof the invention.

FIG. 2 is a schematic cross-section showing an embodiment of anencapsulated electrophoretic display suitable for use with the methodsof the invention.

FIG. 3 illustrates an exemplary equivalent circuit of a single pixel ofan electrophoretic display wherein the voltage on the single pixel iscontrolled with a transistor. The circuit of FIG. 3 is commonly used inactive matrix backplanes.

FIG. 4 illustrates how a positive voltage source and a negative voltagesource can be applied to a top electrode and two separate driveelectrodes to achieve the needed driving voltages at the two separatedrive electrodes.

FIG. 5 is a schematic cross-section showing the positions of the variouscolored particles in an colored electrophoretic medium when displayingblack, white, three subtractive primary colors and three additiveprimary colors.

FIG. 6 shows exemplary push-pull drive schemes for addressing anelectrophoretic medium including three subtractive particles and ascattering (white) particle.

FIG. 7 depicts simplified top plane driving waveforms for the productionof eight colors in an electrophoretic medium including three subtractiveparticles and a scattering (white) particle.

FIG. 8 shows an exemplary drive pattern to achieve a green optical stateat the viewing surface above a first drive electrode and a yellowoptical state at the viewing surface above a second drive electrodeusing only two voltage sources.

FIG. 9A shows the change in L*a*b* values of the eight color indiceswhen the same four particle electrophoretic medium is driven with sevenindependent drive voltages or with two voltage sources and usingcoordinated top electrode voltage cycling.

FIG. 9B shows the date in the graph of FIG. 9A as simulated colors.

DETAILED DESCRIPTION

A system for simplified driving of electrophoretic media using apositive and a negative voltage source, where the voltage sources havedifferent magnitudes, and a controller that cycles the top electrodebetween the two voltage sources and ground while coordinating driving atleast two drive electrodes opposed to the top electrode. The resultingsystem can achieve roughly the same color states as compared tosupplying each drive electrode with six independent drive levels andground. Thus, the system simplifies the required electronics with onlymarginal loss in color gamut. The system is particularly useful foraddressing an electrophoretic medium including four sets of differentparticles, e.g., wherein three of the particles are colored andsubtractive and one of the particles is light-scattering.

The invention provides improved methods of driving electro-optic mediadevices with so-called top-plane switching, i.e., where the voltage onthe top (common) electrode is varied during the course of a deviceupdate. In some embodiments, the invention is used with anelectrophoretic medium including four particles wherein two of theparticles are colored and subtractive and at least one of the particlesis scattering. Typically, such a system includes a white particle andcyan, yellow, and magenta subtractive primary colored particles. In someembodiments, two of the particles and positively-charged and two of theparticles are negatively-charged. In some embodiments, three of theparticles are positively-charged and one of the particles isnegatively-charged. In some embodiments, one of the particles ispositively-charged and three of the particles are negatively-charged.Such a system is shown schematically in FIG. 5 , and it can providewhite, yellow, red, magenta, blue, cyan, green, and black at everypixel.

A display device may be constructed using an electrophoretic fluid ofthe invention in several ways that are known in the prior art. Theelectrophoretic fluid may be encapsulated in microcapsules orincorporated into microcell structures that are thereafter sealed with apolymeric layer. The microcapsule or microcell layers may be coated orembossed onto a plastic substrate or film bearing a transparent coatingof an electrically conductive material. This assembly may be laminatedto a backplane bearing pixel electrodes using an electrically conductiveadhesive. Alternatively, the electrophoretic fluid may be dispenseddirectly on a thin open-cell grid that has been arranged on a backplaneincluding an active matrix of pixel electrodes. The filled grid can thenbe top-sealed with an integrated protective sheet/light-transmissiveelectrode.

Regarding FIGS. 1 and 2 , an electrophoretic display (101, 102)typically includes a top light-transmissive electrode 110, anelectrophoretic medium 120, and bottom drive electrodes 130/135, whichare often pixel electrodes of an active matrix of pixels controlled withthin film transistors (TFT). Alternatively, bottom drive electrodes130/135 may be directly wired to a controller or some other switch thatprovides voltage to the bottom drive electrodes 130/135 to effect achange in the optical state of the electrophoretic medium 120, i.e.,segmented electrodes. Importantly, it is not necessary that a junctionbetween drive electrodes 130/135 corresponds with an intersection ofmicrocapsules or with a wall 127 of a microcell. Because theelectrophoretic medium 120 is sufficiently thin, and the capsules ormicrocells sufficiently wide, the pattern of the drive electrodes(square, circles, hexagons, wavy, text, or otherwise) will show when thedisplay is viewed from the viewing surface; not the pattern of thecontainers. The electrophoretic medium 120 contains at least oneelectrophoretic particle 121, however a second electrophoretic particle122, or a third electrophoretic particle 123, a fourth electrophoreticparticle 124, or more particles is feasible. [It should be noted thatthird electrophoretic particles 123 and fourth electrophoretic particles124 can be included within the microcapsules 126 of FIG. 1 , but havebeen omitted for clarity.] The electrophoretic medium 120 typicallyincludes a solvent, such as isoparaffins, and may also include dispersedpolymers and charge control agents to facilitate state stability, e.g.bistability, i.e., the ability to maintain an electro-optic statewithout inputting any additional energy.

The electrophoretic medium 120 is typically compartmentalized such by amicrocapsule 126 or the walls of a microcell 127. The entire displaystack is typically disposed on a substrate 150, which may be rigid orflexible. The display (101, 102) typically also includes a protectivelayer 160, which may simply protect the top electrode 110 from damage,or it may envelop the entire display (101, 102) to prevent ingress ofwater, etc. Electrophoretic displays (101, 102) may also include one ormore adhesive layers 140, 170, and/or sealing layers 180 as needed. Insome embodiments an adhesive layer may include a primer component toimprove adhesion to the electrode layer 110, or a separate primer layer(not shown in FIGS. 1 or 2 ) may be used. (The structures ofelectrophoretic displays and the component parts, pigments, adhesives,electrode materials, etc., are described in many patents and patentapplications published by E ink Corporation, such as U.S. Pat. Nos.6,922,276; 7,002,728; 7,072,095; 7,116,318; 7,715,088; and 7,839,564,all of which are incorporated by reference herein in their entireties.

Thin-film-transistor (TFT) backplanes usually have only one transistorper pixel electrode or propulsion electrode. Conventionally, each pixelelectrode has associated therewith a capacitor electrode such that thepixel electrode and the capacitor electrode form a capacitor; see, forexample, International Patent Application WO 01/07961. In someembodiments, N-type semiconductor (e.g., amorphous silicon) may be usedto from the transistors and the “select” and “non-select” voltagesapplied to the gate electrodes can be positive and negative,respectively.

As illustrated in in FIG. 3 , each transistor (TFT) is connected to agate line, a data line, and a pixel electrode (propulsion electrode).When there is large enough positive voltage on the TFT gate (or negativedepending upon the type of transistor) then there is low impedancebetween the scan line and pixel electrode coupled to the TFT drain(i.e., Vg “ON” or “OPEN” state), so the voltage on the scan line istransferred to the electrode of the pixel. When there is a negativevoltage on the TFT gate, however, then there is high impedance andvoltage is stored on the pixel storage capacitor and not affected by thevoltage on the scan line as the other pixels are addressed (i.e., Vg“OFF” or “CLOSED”), Thus, ideally, the TFT should act as a digitalswitch. In practice, there is still a certain amount of resistance whenthe TFT is in the “ON” setting, so the pixel takes some time to charge.Additionally, voltage can leak from V_(S) to V_(pix) when the TFT is inthe “OFF” setting, causing cross-talk. Increasing the capacitance of thestorage capacitor C_(s) reduces cross-talk, but at the cost of renderingthe pixels harder to charge, and increasing the charge time. As shown inFIG. 3 , a separate voltage (V_(TOP)) is provided to the top electrode,thus establishing an electric field between the top electrode and thepixel electrode (V_(FPL)). Ultimately, it is the value of V_(FPL) thatdetermines the optical state of the relevant electro-optic medium. Whilea first side of the storage capacitor is coupled to the pixel electrode,a second side of the storage capacitor is coupled to a separate line(V_(COM)) that allows the charge to be removed from the pixel electrode.See, for example, U.S. Pat. No. 7,176,880, which is incorporated byreference in its entirety. [In some embodiments, N-type semiconductor(e.g., amorphous silicon) may be used to from the transistors and the“select” and “non-select” voltages applied to the gate electrodes can bepositive and negative, respectively.] In some embodiments V_(COM) may begrounded, however there are many different designs for draining chargefrom the charge capacitor, e.g., as described in U.S. Pat. No.10,037,735, which is incorporated by reference in its entirety.

One problem with conventional amorphous silicon TFTs is that theoperating voltage is limited to roughly ±15V, whereupon the transistorsstart to leak current and ultimately fail. While the operating range of±15V is suitable for many two-particle electrophoretic systems, it hasbeen found that having increased voltage ranges makes it easier toseparate particles with different zeta potentials, resulting in advancedelectrophoretic displays that update faster and have more reproduciblecolors. One solution for increasing the voltage range to a pixelelectrode is to use top plane switching, i.e. whereby the voltage on thetop (common) electrode is varied as a function of time.

The principle of top plane switching is illustrated in FIG. 4 . Anexemplary electrophoretic display 401 includes an electrophoretic medium420 disposed between a top electrode 410 and a (bottom) drive electrode430. The electrophoretic medium 420 in FIG. 4 is shown with fourdifferent types of electrophoretic particles, however theelectrophoretic medium 420 can have fewer types of different particlesor more types of different particles than shown. In the simplifiedembodiment of FIG. 4 , both the top electrode 410 and the driveelectrode 430 are supplied by two different power supplies 440 and 460,which could be from the same power source (not shown). In addition aground voltage 470 is available. Typically one power supply is positivewith respect to ground and one power supply is negative with respect toground. Which power supply (or ground) is connected to which electrodeat a given unit of time (a frame) is controlled by a controller 470. Thecontroller can be a commercial electrophoretic display controller suchas manufactured by UltraChip, or it can be a research controller such asoffered by E Ink Corporation (HULK Controller, ARC30™ controller) or itcan be a virtual controller using, e.g., LABVIEW® to control the outputof a voltage board.

As illustrated in the equations below the electrophoretic display 401 ofFIG. 4 , each combination of voltage provided to the top electrode 410and the drive electrode 430 results in a voltage differential ofΔV=V(Drive Electrode)−V(Top Electrode) on the electrophoretic medium420. As can be seen by the equations (and as discussed below), bymodifying the voltage on the top electrode, a larger dynamic range ofvoltage on the electrophoretic medium 420 can be achieved. Additionally,where the magnitude of the 440 and 460 are different, intermediatedifferential voltage values on the electrophoretic medium can beachieved. As shown in FIG. 4 , by carefully coordinating when the topelectrode 410 and the drive electrode 430 are connected to which powersupply, seven different voltages are available to the electrophoreticmedium 420.

While FIG. 4 illustrates only a single drive electrode 430, it isunderstood that the principle can be extended to a system with manydrive pixels, such as available with an active matrix backplane.However, coordinating the necessary top electrode voltage to achieve adesired voltage differential across a particular pixel becomescomplicated very quickly as the number of pixels increases. In practice,top plane switching with an active matrix backplane uses independentvoltage controllers for the top plane and the pixel electrodes, andrequires top electrode voltage cycles that last many frames while theindividual pixel electrodes are switched to produce the desiredwaveforms. More details of this method are described in U.S. Pat. No.10,593,272, which is incorporated by reference in its entirety.

In the instance of ACeP®, each of the eight principal colors (red,green, blue, cyan magenta, yellow, black and white) corresponds to adifferent arrangement of the four pigments, such that the viewer onlysees those colored pigments that are on the viewing side of the whitepigment (i.e., the only pigment that scatters light). More specifically,when the cyan, magenta and yellow particles lie below the whiteparticles (Situation [A] in FIG. 5 ), there are no particles above thewhite particles and the pixel simply displays a white color. When asingle particle is above the white particles, the color of that singleparticle is displayed, yellow, magenta and cyan in Situations [B], [D]and [F] respectively in FIG. 5 . When two particles lie above the whiteparticles, the color displayed is a combination of those of these twoparticles; in FIG. 5 , in Situation [C], magenta and yellow particlesdisplay a red color, in Situation [E], cyan and magenta particlesdisplay a blue color, and in Situation [G], yellow and cyan particlesdisplay a green color. Finally, when all three colored particles lieabove the white particles (Situation [H] in FIG. 5 ), all the incominglight is absorbed by the three subtractive primary colored particles andthe pixel displays a black color.

It is possible that one subtractive primary color could be rendered by aparticle that scatters light, so that the display would comprise twotypes of light-scattering particle, one of which would be white andanother colored. In this case, however, the position of thelight-scattering colored particle with respect to the other coloredparticles overlying the white particle would be important. For example,in rendering the color black (when all three colored particles lie overthe white particles) the scattering colored particle cannot lie over thenon-scattering colored particles (otherwise they will be partially orcompletely hidden behind the scattering particle and the color renderedwill be that of the scattering colored particle, not black). It wouldnot be easy to render the color black if more than one type of coloredparticle scattered light.

It has been found that waveforms to sort the four pigments intoappropriate configurations to make these colors are best achieved withat least seven voltage levels (high positive, medium positive, lowpositive, zero, low negative, medium negative, high negative). FIG. 6shows typical waveforms (in simplified form) used to drive afour-particle color electrophoretic display system described above. Suchwaveforms have a “push-pull” structure: i.e., they consist of a dipolecomprising two pulses of opposite polarity. The magnitudes and lengthsof these pulses determine the color obtained. In general, the higher themagnitude of the “high” voltages, the better the color gamut achieved bythe display. The “high” voltage is typically between 20V and 30V, moretypically around 25V, e.g., 24V The “medium” (M) level is typicallybetween 10V and 20V, more typically around 15V, e.g., 15V or 12V. The“low” (L) level is typically between 3V and 10V, more typically around7V, e.g., 9V or 5V. Of course, the values for H, M, L will dependsomewhat on the composition of the particles, as well as the environmentof the electrophoretic medium. In some applications, H, M, L may be setby the cost of the components for producing and controlling thesevoltage levels.

As shown in FIG. 6 , if the top electrode is held at a constant voltage(i.e., not top plane switched), even “simple” waveforms for the ACeP®system require that the driving electronics provide seven differentvoltages to the data lines during the update of a selected pixel of thedisplay (+H, +M, +L, 0, −L, −M, −H ). While multi-level source driverscapable of delivering seven different voltages are available, mostcommercially-available source drivers for electrophoretic displayspermit only three different voltages to be delivered during a singleframe (typically a positive voltage, zero, and a negative voltage).

Of course, achieving the desired color with the driving pulses of FIG. 6is contingent on the particles starting the process from a known state,which is unlikely to be the last color displayed on the pixel.Accordingly, a series of reset pulses precede the driving pulses, whichincreases the amount of time required to update a pixel from a firstcolor to a second color. The reset pulses are described in greaterdetail in U.S. Pat. No. 10,593,272, incorporated by reference. Thelengths of these pulses (refresh and address) and of any rests (i.e.,periods of zero voltage between them may be chosen so that the entirewaveform (i.e., the integral of voltage with respect to time over thewhole waveform) is DC balanced (i.e., the integral of voltage over timeis substantially zero). DC balance can be achieved by adjusting thelengths of the pulses and rests in the reset phase so that the netimpulse supplied in the reset phase is equal in magnitude and oppositein sign to the net impulse supplied in the address phase, during whichphase the display is switched to a particular desired color.

In addition, the foregoing discussion of the waveforms, and specificallythe discussion of DC balance, ignores the question of kickback voltage.In practice, as previously, every backplane voltage is offset from thevoltage supplied by the power supply by an amounts equal to the kickbackvoltage V_(KB). Thus, if the power supply used provides the threevoltages +V, 0, and −V, the backplane would actually receive voltagesV+V_(KB), V_(KB), and −V+V_(KB) (note that V_(KB), in the case ofamorphous silicon TFTs, is usually a negative number). The same powersupply would, however, supply +V, 0, and −V to the front electrodewithout any kickback voltage offset. Therefore, for example, when thefront electrode is supplied with −V the display would experience amaximum voltage of 2V+V_(KB) and a minimum of V_(KB). Instead of using aseparate power supply to supply V_(KB) to the front electrode, which canbe costly and inconvenient, a waveform may be divided into sectionswhere the front electrode is supplied with a positive voltage, anegative voltage, and V_(KB). In addition to the kickback

Higher Voltage Addressing with Metal Oxide Backplanes

While modifying the rail voltages provides some flexibility in achievingdiffering electro-optical performance from a four-particleelectrophoretic system, there are many limitations introduced bytop-plane switching. For example, it is typically preferred, in order tomake a white state with displays of the present invention, that thelower negative voltage V_(M−) is less than half the maximum negativevoltage V_(H−). As shown in the equations above, however, top-planeswitching requires that the lower positive voltage is always at leasthalf the maximum positive voltage, typically more than half.

An alternative solution to the complications of top-plane switching canbe provided by fabricating the control transistors from less-commonmaterials that have a higher electron mobility, thereby allowing thetransistors to switch larger control voltages, for example +/−30V,directly. Newly-developed active matrix backplanes may include thin filmtransistors incorporating metal oxide materials, such as tungsten oxide,tin oxide, indium oxide, and zinc oxide. In these applications, achannel formation region is formed for each transistor using such metaloxide materials, allowing faster switching of higher voltages. Suchtransistors typically include a gate electrode, a gate-insulating film(typically SiO₂), a metal source electrode, a metal drain electrode, anda metal oxide semiconductor film over the gate-insulating film, at leastpartially overlapping the gate electrode, source electrode, and drainelectrode. Such backplanes are available from manufacturers such asSharp/Foxconn, LG, and BOE.

One preferred metal oxide material for such applications is indiumgallium zinc oxide (IGZO). IGZO-TFT has 20-50 times the electronmobility of amorphous silicon. By using IGZO TFTs in an active matrixbackplane, it is possible to provide voltages of greater than 30V via asuitable display driver. Furthermore, a source driver capable ofsupplying at least five, and preferably seven levels provides adifferent driving paradigm for a four-particle electrophoretic displaysystem. In an embodiment, there will be two positive voltages, twonegative voltages, and zero volts. In another embodiment, there will bethree positive voltages, three negative voltages, and zero volts. In anembodiment, there will be four positive voltages, four negativevoltages, and zero volts. These levels may be chosen within the range ofabout −27V to +27V, without the limitations imposed by top planeswitching as described above.

Using advanced backplanes, such as metal oxide backplanes, it ispossible to directly address each pixel with a suitable push-pullwaveform, i.e., as described in FIG. 6 . This greatly reduces the timerequired to update each pixel, in some instances transforming asix-second update to less than one second. While, in some cases, it maybe necessary to use reset pulses to establish a starting point foraddressing, the reset can be done quicker at higher voltages.Additionally, in four-color electrophoretic displays having reducedcolor sets, it is possible to directly drive from a first color to asecond color with a specific waveform that is only slightly longer thanthe push-pull waveforms shown in FIG. 6 .

Simplified Top-Plane Switching

To reduce the length of time and flashiness of an update, the complexityof the front-plane switching can be reduced in exchange for a smallernumber of available colors. Additionally, because the particles have afinite speed within the electrophoretic medium, the amount of time forwhich the dipole is applied also influences the size of the color gamut.

FIG. 7 shows such a solution in which a simplified top plane switchingpulse sequence is used (top left panel), with simplified backplane pulsesequences (left; below) being matched to the single top-plane sequence,thereby providing at least distinct colors. The top plane is switchedbetween two voltages, one positive and one negative, while the backplane can take three different voltages: positive, negative, and zero.(In FIG. 7 , the voltage levels are relative, i.e., 1, 0, −1, but wouldin many instances actually be 15V, 0, and −15V as is typically withcommercial backplanes including amorphous silicon thin filmtransistors.) Note that by subtracting the pulse sequence of thetop-plane from the backplane pulse sequence (FIG. 7 left), the eightcolor sequences in FIG. 6 are achieved (FIG. 7 right). It is understoodthat for the pulse sequences in FIG. 6 and FIG. 7 , the electrophoreticfluid includes a white pigment that is negatively charged, a magentapigment and a cyan pigment that are positively charged, and the yellowpigment may be either positively or negatively charged, or essentiallyneutral. Other color/charge combinations are possible and the waveformscan be adjusted accordingly.

As discussed previously, in the waveforms of FIG. 7 at least fivedifferent voltages are required. In an active matrix drivingenvironment, this may be achieved either (a) by supplying a choice offive different voltages to the columns when a particular row is selectedat a particular time, or (b) by providing a choice of fewer (say, three)different voltages to the columns when a particular row is selected at afirst time, and a different set of voltages when the same row isselected at a second time, or (c) by providing the same choice of threevoltages to the columns at both the first and second times, but changingthe potential of the front electrode between the first and second times.Option (c) is particularly helpful when at least one of the voltagesrequired to be supplied is higher than the backplane electronics cansupport.

Because, with top plane switching, it is not possible to assert a highpositive and a high negative potential simultaneously, it is necessaryto offset the +/− dipoles of the top plane with respect to the −/+dipoles of the backplane. In the waveform shown in FIG. 7 , there isonly one dipole per transition. This provides the least “flashy”waveform possible, since each dipole results in two visible opticalchanges to the display. In cases where five different voltage levels canbe supplied to the backplane electrodes when each row is selected, andwhere the backplane electronics can support the highest voltages needed,it is not necessary to offset the dipoles in the manner shown in FIG. 7.

Driving with Cycled Top Plane Voltage

For the drive sequences of FIG. 7 , the voltages applied to the topplane are denoted V_(t+) and V_(t−), respectively, and those applied tothe back plane V_(b+) and V_(b−), respectively, and|V_(t+)|=|V_(t−)|=|V_(b+)|=|B_(b−)|=V. Accordingly, when the maximumsupply voltage is +/−15 volts, as is typical with commercial backplanes,the voltages across the electrophoretic medium become 30V, 28V, 0V,−28V, and −30V.

The maximum voltage magnitudes (i.e., “rail”) of the top-plane electrodeand the back-plane electrode need not be the same, however. For example,rail voltages offsets can be calculated from some nominal maximumvoltage magnitude value, V. The offset for each rail may be denoted w,x, y and z, while it is assumed that the zero voltage rail is kept atzero and not applied to the top plane.

Thus:

V _(t+) =V+w

V _(t−) =−V+x

V_(t0)=0

V _(b+) =V+y

V _(b−) =−V+z

V_(b0)=0

Referenced to the backplane voltage, three different negative voltagesof high, medium and low magnitudes may be applied to the electrophoreticmedium when the top plane is set to V_(t+), denoted as V_(H−), V_(M−),and V_(L−), (i.e., V_(b)−V_(t), where V_(b) can take any of the threevalues shown above).

These voltages are:

V _(H−)=−2V+z−w

V _(M−) =−V−w

V _(L−) =y−w

The voltages available when the top plane is set to V_(t−) are:

V _(H+)=2V+y−x

V _(M+) =V−x

V _(L+) =z−x

The voltages available when the top plane is set to 0 are:

V _(H0) =V+y

V_(M0)=0

V _(L0) =−V+z

It is apparent that when w=y and x=z it is possible to maintain the zerovoltage condition whether the top plane is set to V_(t+), V_(t−), orzero. In practice, waveforms require significantly greater complexityand length if optimum colors are to be obtained. Accordingly, the topplane switching pattern require thus be significantly more complex theone illustrated in FIG. 7 . A difficulty arises, however, inapplications requiring simultaneous updates in different regions of adisplay with staggered start times separated by less than the length ofone waveform. Because the top plane potential is asserted over theentire display it may be impossible to initiate a new update in oneregion of the display before the end of a previously-initiated update inanother location.

The problem of coordinating multiple simultaneous updates each requiringtop plane switching can be solved by cycling the top plane voltage whilestretching out the waveform, as illustrated in FIG. 8 . (V_(TE)=topelectrode voltage, V_(DE1)=first drive electrode voltage, V_(DE2)=seconddrive electrode voltage, ΔV_(DE1)=voltage differential onelectrophoretic medium between first drive electrode and top electrode,ΔV_(DE2)=voltage differential on electrophoretic medium between seconddrive electrode and top electrode.) A green waveform and a yellowwaveform, previously created for a seven-level backplane capable ofproviding +/−24V, +/−15V, +/−9V or 0V at any pixel location in anyframe, was modified for cycled top plane driving. A controller providessuccessive frames of +15V, −9V and 0V (i.e., V=15V, w=y=0V and x=z=6V inthe above equations) to the top electrode, as shown in FIG. 9 . Bystretching out the waveform, and coordinating the voltage to the firstand second drive electrodes with the top electrode cycle, it waspossible to effect simultaneous color updates at two different driveelectrodes using top-plane switching.

When the top electrode is at +15V the voltage differential available tothe electrophoretic medium is −24V, −15V, −0V. When the top electrode isat −9V, the voltage differential available to the electrophoretic mediumis 24V 9V and 0V. When the top electrode is at ground (0V), the voltagedifferential available to the electrophoretic medium is 15V, 0V and −9Vin the third. [By convention, the voltage differential is ΔV=V(DriveElectrode)−V(Top Electrode).] Thus, 7 voltage levels were available:+/−24V, +/−15V and +/−9V plus 0V. It should be noted that when aparticular drive electrode needs to “wait” for the next top electrodeframe, that drive electrode is set to the same voltage as the topelectrode so that the voltage differential across the electrophoreticmedium is zero for that frame. Obviously, this makes the waveformslonger in time, and each “simple” waveform now requires three timeslonger updates than the original multilevel waveform.

Using a model of a four-particle electrophoretic system, the topelectrode cycled driving with +15V, −9V, and 0 was tested against thesame system having seven individual drive levels and a static topelectrode. The results are shown in Tables 1 and 2, below, andrepresented in the graph of FIG. 9A and the simulated color table ofFIG. 9B.

TABLE 1 Calculated L*a*b* values for modded ACeP system using dedicatedseven-level driver. color L* a* b* Color Black 20.4 0.3 −17.5 Black Blue37.3 −0.4 −22.3 Blue Red 49.4 18 8 Red Magenta 39.5 28.5 −11.3 MagentaGreen 52.9 −14.5 11.6 Green Cyan 44.9 −12.7 −9.2 Cyan Yellow 64.5 −8.636.1 Yellow White 67.2 −6.7 15.2 White Gamut 14565 CR 11.9

TABLE 2 Calculated L*a*b* values for modeled ACeP system using topelectrode cycling and +15 V and −9 V power supply. color L* a* b* ColorBlack 18.1 −2.5 −6.6 Black Blue 30.5 −12.7 −15.8 Blue Red 48.5 10.9 14.6Red Magenta 38 26.3 −7 7 Magenta Green 46 −21.4 7.9 Green Cyan 35 −18−13.6 Cyan Yellow 59.7 −8.9 27.9 Yellow White 61.7 −2.8 1.7 White Gamut15991 CR 11.8

Comparing Tables 1 and 2, it seems that there is little penalty for thetop electrode cycling beyond the longer update times. In fact the colorgamut (color space) is actually slightly larger for the top electrodecycling method. The differences between the two methods can be furthervisualized by considering FIGS. 9A and 9B. In FIGS. 9A, the filledcircles represent the L*a*b* measurement of the seven-level driver,whereas the open circles represent the L*a*b* measurement of the cycledtop electrode driving. As can be seen from FIGS. 9A and 9B, theresulting primary color states are quite similar. (Compare positions ofopen circles to filled circles.) The greatest change is seen in thegreen primary (left center of FIG. 9A) where the green primary driftsquite a bit toward the yellow. The difference in color states for thegreen primary is also evident in FIG. 9B.

Thus, the invention provides for full color electrophoretic displaysthat are capable of directly addressing the electrophoretic medium withand without top plane switching, as well as waveforms for suchelectrophoretic displays. Having thus described several aspects andembodiments of the technology of this application, it is to beappreciated that various alterations, modifications, and improvementswill readily occur to those of ordinary skill in the art. Suchalterations, modifications, and improvements are intended to be withinthe spirit and scope of the technology described in the application. Forexample, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the embodiments described herein. Thoseskilled in the art will recognize, or be able to ascertain using no morethan routine experimentation, many equivalents to the specificembodiments described herein. It is, therefore, to be understood thatthe foregoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, kits, and/or methods described herein, ifsuch features, systems, articles, materials, kits, and/or methods arenot mutually inconsistent, is included within the scope of the presentdisclosure.

1. A system for driving an electrophoretic medium, comprising: anelectrophoretic display including, a light-transmissive top electrode ata viewing surface, a first drive electrode, a second drive electrode, anelectrophoretic medium disposed between the top electrode and the firstand second drive electrodes; a power source capable of providing apositive voltage and a negative voltage, where the magnitude of thepositive voltage and the negative voltage are different; a controllercoupled to the top electrode driver, the first drive electrode driver,and the second drive electrode driver, the controller configured toprovide: in a first frame, the positive voltage to the top electrode,the negative voltage to the first drive electrode, and the positivevoltage to the second drive electrode, in a second frame, the negativevoltage to the top electrode, the negative voltage to the first driveelectrode, and the negative voltage to the second drive electrode, in athird frame, the ground voltage to the top electrode, the ground voltageto the first drive electrode, and the positive voltage to the seconddrive electrode, and in a fourth frame, the positive voltage to the topelectrode, the positive voltage to the first drive electrode, and thepositive voltage to the second drive electrode.
 2. The system of claim1, wherein the controller is configured to further provide: in a fifthframe, the negative voltage to the top electrode, the ground voltage tothe first drive electrode, and the negative voltage to the second driveelectrode, and in a sixth frame, the ground voltage to the topelectrode, the ground voltage to the first drive electrode, and theground voltage to the second drive electrode.
 3. The system of claim 1,wherein the electrophoretic medium is encapsulated in a plurality ofmicrocapsules and the microcapsules are dispersed in a polymer binderbetween the top electrode and the first and second drive electrodes. 4.The system of claim 1, wherein the electrophoretic medium isencapsulated in an array of microcells having openings wherein theopening are sealed with a polymer binder, and the array of microcells isdisposed between the top electrode and the first and second driveelectrodes.
 5. The system of claim 1, wherein the electrophoretic mediumcomprises a non-polar fluid and four sets of particles having differentoptical properties.
 6. The system of claim 5, wherein the first andsecond sets of particles bear charges of opposite polarity, the thirdand fourth sets of particles bear charges of opposite polarity, thefirst particle is a light-scattering particle, and the second, third,and fourth sets of particles are each a subtractive primary colordifferent from each other.
 7. The system of claim 6, wherein thecontroller is configured to provide combinations of the positivevoltage, the negative voltage, and the ground voltage to the topelectrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can be displayed atthe viewing surface.
 8. The system of claim 5, wherein the first andsecond sets of particles bear charges of opposite polarity, the thirdand fourth sets of particles bear the same charge as the secondparticle, the first particle is a light-scattering particle, and thesecond, third, and fourth sets of particles are each a subtractiveprimary color different from each other.
 9. The system of claim 8,wherein the controller is configured to provide combinations of thepositive voltage, the negative voltage, and the ground voltage to thetop electrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can be displayed atthe viewing surface.
 10. The system of claim 1, wherein the positivevoltage is +15V and the negative voltage is −9V.
 11. The system of claim1, wherein the positive voltage is +9V and the negative voltage is −15V.17. A system for driving an electrophoretic medium, comprising: anelectrophoretic display including, a light-transmissive top electrode ata viewing surface, a first drive electrode, a second drive electrode, anelectrophoretic medium disposed between the top electrode and the firstand second drive electrodes; a power source capable of providing apositive voltage and a negative voltage, where the magnitude of thepositive voltage and the negative voltage are different; a controllercoupled to the top electrode driver, the first drive electrode driver,and the second drive electrode driver, the controller configured toprovide: in a first frame, the positive voltage to the top electrode,the negative voltage to the first drive electrode, and the positivevoltage to the second drive electrode, in a second frame, the negativevoltage to the top electrode, the negative voltage to the first driveelectrode, and the negative voltage to the second drive electrode, in athird frame, the ground voltage to the top electrode, the ground voltageto the first drive electrode, and the ground voltage to the second driveelectrode, and in a fourth frame, the positive voltage to the topelectrode, the positive voltage to the first drive electrode, and thepositive voltage to the second drive electrode.
 13. The system of claim12, wherein the controller is configured to further provide: in a fifthframe, the negative voltage to the top electrode, the ground voltage tothe first drive electrode, and the negative voltage to the second driveelectrode, and in a sixth frame, the ground voltage to the topelectrode, the ground voltage to the first drive electrode, and theground voltage to the second drive electrode.
 14. The system of claim12, wherein the electrophoretic medium is encapsulated in a plurality ofmicrocapsules and the microcapsules are dispersed in a polymer binderbetween the top electrode and the first and second drive electrodes. 15.The system of claim 12, wherein the electrophoretic medium isencapsulated in an array of microcells having openings wherein theopening are sealed with a polymer binder, and the array of microcells isdisposed between the top electrode and the first and second driveelectrodes.
 16. The system of claim 12, wherein the electrophoreticmedium comprises a non-polar fluid and four sets of particles havingdifferent optical properties.
 17. The system of claim 16, wherein thefirst and second sets of particles bear charges of opposite polarity,the third and fourth sets of particles bear charges of oppositepolarity, the first particle is a light-scattering particle, and thesecond, third, and fourth sets of particles are each a subtractiveprimary color different from each other.
 18. The system of claim 17,wherein the controller is configured to provide combinations of thepositive voltage, the negative voltage, and the ground voltage to thetop electrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can be displayed atthe viewing surface.
 19. The system of claim 16, wherein the first andsecond sets of particles bear charges of opposite polarity, the thirdand fourth sets of particles bear the same charge as the secondparticle, the first particle is a light-scattering particle, and thesecond, third, and fourth sets of particles are each a subtractiveprimary color different from each other.
 20. The system of claim 19,wherein the controller is configured to provide combinations of thepositive voltage, the negative voltage, and the ground voltage to thetop electrode and the first drive electrode such that the colors white,yellow, red, magenta, blue, cyan, green, and black can he displayed atthe viewing surface.